Cache 的写缺失怎么理解呢

Cache 的读缺失可以理解，可是写缺失呢，是指cpu往cache里面写，但是cache里面已经没有空间了，要按照LRU算法替换掉一些信息么？

write back：CPU更新cache时，只是把更新的cache区标记一下，并不同步更新memory。只是在cache区要被新进入的数据取代时，才更新memory。这样做的原因是考虑到很多时候cache存入的是中间结果，没有必要同步更新memory。优点是CPU执行的效率提高，缺点是实现起来技术比较复杂。

Write-misses写缺失的处理方式
对于写操作，存在写入缓存缺失数据的情况，这时有两种处理方式：
Write allocate
(aka
Fetch on write) – Datum at the missed-write location is loaded to cache, followed by a write-hit operation. In this approach, write misses are similar to read-misses.
No-write allocate
(aka
Write-no-allocate,
Write around) – Datum at the missed-write location is not loaded to cache, and is written directly to the backing store. In this approach, actually only system reads are being cached.
Write allocate方式将写入位置读入缓存，然后采用write-hit（缓存命中写入）操作。写缺失操作与读缺失操作类似。
No-write allocate方式并不将写入位置读入缓存，而是直接将数据写入存储。这种方式下，只有读操作会被缓存。
无论是Write-through还是Write-back都可以使用写缺失的两种方式之一。只是通常Write-back采用Write allocate方式，而Write-through采用No-write allocate方式；因为多次写入同一缓存时，Write allocate配合Write-back可以提升性能；而对于Write-through则没有帮助。